1. Field of the Invention
The present invention relates to a method for accurately locating nearly vertical subsurface reflectors and more particularly to a method of using common reflection point gathers to quickly derive the accurate dip and location of a nearly vertical reflector or subsurface fault.
2. Related Prior Art
Depth migration iteratively constructs a model which reproduces the traveltimes observed on individual traces. That model is then used to migrate the data to produce a stacked depth section. It is well known that the quality of the model is important for successful depth migration. Conventional migration velocity analysis examines the migrated image for hints to correct the velocity model. Near vertical boundaries such as faults or salt flanks must be accurately positioned to produce reliable images of deeper reflectors. At present, they are placed by trial and error, since a depth image of a nearly vertical boundary is difficult to produce. If the errors caused by misplaced fault locations can be corrected, the result is a better depth image from better aligned events and a better correction to the velocities from a follow-up tomograph using the velocity tomography. Examples of seismic data processing methods which include migration and examples of methods for determining the location of subsurface interfaces are as follows.
U.S. Pat. No. 4,241,429 entitled "Velocity Determination and Stacking Process from Seismic Exploration of Three Dimensional Reflection Geometry" (Marvin G. Bloomquist et al) relates to a method for determining the dip and strike of subsurface interfaces and average propagation velocity of seismic waves. In seismic exploration, linear, multiple fold, common depth point sets of seismograms with three dimensional reflection geometry are used to determine the dip and strike of the subsurface reflecting interfaces and the average velocity of the path of the seismic energy to the reflecting interface. The reflections with each set appear with time differences on a hyperbola with trace spacing determined by the source receiver coordinate distance along the lines of exploration. The offset of the apex of this hyperbola is determined from a normal moveout velocity search of the type performed on two dimensional common depth point sets. This search identifies the correct stacking velocity and hyperbola offset which are used to determine dip, strike and average velocity.
U.S. Pat. No. 4,736,347 entitled "Multiple Stacking and Spatial Mapping of Seismic Data" (Bernard Goldberg et al) relates to a method for determining the dip of subsurface formations and the apparent acoustic velocity. Seismic traces are stacked in a plurality of orthogonal measures to form multiple stack traces at a positive offset. This stacking process determines the apparent velocities as functions of the travel time at the positive offset. The interval acoustic velocity of the first layer is then determined from knowledge of surface topography, source-receiver offset, two-way travel times and the first reflector apparent velocities. The first layer velocity information enables the incident and emergent angles of the raypaths at the surface to be calculated, as well as enabling the dip angles and spatial coordinates of the reflection points on the first reflecting boundary to be determined. Seismic data corresponding to the second reflecting boundary are then mapped spatially to the first reflecting boundary by ray tracing and by calculating the apparent velocities at the first boundary. The process is repeated for each succeedingly deeper boundary. The derived acoustic velocity model of the earth is displayed as a stacked seismic section in spatial coordinates.
U.S. Pat. No. 4,813,027 entitled "Method and Apparatus for Enhancing Seismic Data" (Hans Tieman) relates to a method and apparatus for stacking a plurality of seismic midpoint gathers to provide a pictorial representation of seismic events. The approximate propagation velocity, corresponding to a selected event in a common midpoint gather, is determined by summing the common midpoint gather using first and second weights to provide respective first and second weighted sums over an offset based on an estimated velocity corresponding to the event. A velocity error value indicative of the approximate error between the estimated velocity and the actual velocity is developed from the sums. The common midpoint gather is then re-stacked in accordance with the determined propagation velocity to provide an enhanced pictorial representation of the seismic event. The first and second weighted sums are taken over a time window centered upon an estimated zero offset travel time for the event. The first and second weights can be selected to provide rapid, slow or intermediate convergence upon the true velocity. The velocity error value is determined as a function of the derivation of the peak of the first weighted sum from the center of the time window, relative to the derivation of the peak of the second weighted sum from the center of the time window. Alternatively, the velocity error value is determined as a function of the derivation of the peak of the cross-correlation of the first and second weighted sums from the center of the time window.
U.S. Pat. No. 4,766,574 "Method for Depth Imaging Multicomponent Seismic Data" (Norman D. Whitmore, Jr. et al) relates generally to a method of geophysical exploration. This method may be used for imaging multi-component seismic data to obtain depth images of the earth's subsurface geological structure as well as estimates of compressional and shear wave interval velocities. In particular, measures are obtained of imparted seismic wavefields incident on reflecting interfaces in the earth's subsurface and of resulting seismic wavefields scattered therefrom. The incident and scattered wavefields are employed to produce time-dependent reflectivity functions which are representative of the reflecting interfaces. By migrating the time-dependent reflectivity functions, better depth images of the reflecting interfaces can be obtained.
U.S. Pat. No. 4,802,147 entitled "Method For Segregating And Stacking Vertical Seismic Profile Data In Common Reflection Point Bins" (George P. Moeckel) relates to a method for segregating and stacking vertical seismic profile data. The offset difference between the well location and the position of the source is divided into equal segments. Vertical seismic profile moveout corrected data is placed in common reflection point bins and stacked.
U.S. Pat. No. 4,802,146 titled "Method For Moveout Correction And Stacking Velocity Estimation Of Offset VSP Data" (George P. Moeckel) relates to a moveout correction process and velocity stacking estimation process to permit stacking of vertical seismic profile (VSP) data. The primary reflection time is determined by using the two-way travel time, the root mean square velocity of acoustic pulses in the formation and the first arrival time of direct path acoustic pulses.
U.S. Pat. No. 4,992,996 titled "Interval Velocity Analysis And Depth Migration Using Common Reflection Point Gathers" (Wang et al) relates to a method for performing velocity analysis while eliminating the effects on weak signals caused by strong signals which includes migrating each event of the prestack trace to a single location instead of all possible locations. The input trace is divided into many windows, and each window is migrated to a place determined by ray tracing the center of the window through the model. If the model is accurate, each event will be migrated to the proper location yielding an accurate depth section with no migration artifacts. As a by product, if the model is not accurate, the post migrated parts, migrated common offset depth sections sorted into common midpoint gathers provide an interpretable velocity analysis.
U.S. Pat. No. 5,089,994 titled "Tomographic Estimation Of Seismic Transmission Velocities From Constant Offset Depth Migrations" (Harlan et al) relates to a method for improving velocity models so that constant offset migrations estimate consistent positions for reflectors which includes tomographic estimation of seismic transmission velocities from constant offset depth migrations. A method of converting inconsistencies in reflector positioning from constant offset migrations into equivalent errors in modeled traveltimes is introduced, so that conventional methods of traveltime tomography can improve the velocity model.